1. Field of the Invention
This invention relates to amplifiers having a gain factor controllable with great precision and a programmable or controllable filter wherein the control is provided by a digital to analog converter (DAC).
2. Brief Description of the Prior Art
Amplifiers are often required which are capable of providing multiple ranges of amplification wherein the range to be utilized can be controlled. Such prior art multiple range amplifiers have been limited to a small number of ranges, typically three, such amplifiers not being easily adjusted to a binary scheme. In addition, such prior art multiple range amplifiers have been built using discrete components including an operational amplifier, a multiplexer to select the proper resistors corresponding to the selected range and an output amplifier. The number of ranges is limited by the size (N) of the multiplexer and the number of resistors, usually precision, is N+2 (number of ranges plus 2). One problem encountered with such prior art amplifiers is that each of the resistors in the resistor chain, by which the amplification range is determined, must be a precision resistor to provide a high degree of accuracy and such resistors are very costly and require high installation costs. Furthermore, even the costly resistors obtainable may not provide the degree of accuracy required and are also not readily available. A further problem is that, in the event the resistors do not provide the degree of accuracy required, there is no practical way to adjust for minor errors. Accordingly, the degree of accuracy continually obtainable with such prior art amplifiers has been less than is desired for many operations.
In automatic test equipment, measurement of low amplitude DC signals is common. Often these signals contain unwanted AC signals, generally described as "noise". Failure to remove these "noise" signals causes unstable and erroneous measurements. The use of "high efficiency" switching power supplies in test systems has added greatly to the noise problems. Due to the switching action, electrical noise in the range of 30 Hz to 50 KHz is introduced and is generally removed by some form of filtering action. Test systems that provide an averaging function generally use one of two methods. In one method, many individual readings are taken and the average is computed. These multiple readings can require considerable CPU time. In the second method, a multi-range time interval integrator circuit is used. This circuit requires a large number of analog components and a programmable counter/timer. The analog portion of the circuit controls the voltage and time constant scaling while the counter controls the integration period. With this arrangement, the time constant and voltage scaling are not independent. This lack of independence prevents the use of the long time constants on the lower voltage signals.